In a cellular communication system, a geographical region is divided into a number of cells each of which is served by a base station. The base stations are interconnected by a fixed network which can communicate data between the base stations. A mobile station is served via a radio communication link by the base station of the cell within which the mobile station is situated. Communication from a mobile station to a base station is known as uplink, and communication from a base station to a mobile station is known as downlink.
The fixed network interconnecting the base stations is operable to route data between any two base stations, thereby enabling a mobile station in a cell to communicate with a mobile station in any other cell. In addition, the fixed network comprises gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN), thereby allowing mobile stations to communicate with landline telephones and other communication terminals connected by a landline. Furthermore, the fixed network comprises much of the functionality required for managing a conventional cellular communication network including functionality for routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.
Currently, the most ubiquitous cellular communication system is the 2nd generation cellular communication system known as the Global System for Mobile communication (GSM). Further description of the GSM TDMA communication system can be found in ‘The GSM System for Mobile Communications’ by Michel Mouly and Marie Bernadette Pautet, Bay Foreign Language Books, 1992, ISBN 2950719007.
Currently, 3rd generation systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD). In CDMA systems, user separation is obtained by allocating different spreading and scrambling codes to different users on the same carrier frequency and in the same time intervals. In TDD, user separation is achieved by assigning different time slots to different uses in a similar way to TDMA. However, in contrast to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA and specifically of the Wideband CDMA (WCDMA) mode of UMTS can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In a UMTS CDMA communication system, the communication network comprises a core network and a Radio Access Network (RAN). The core network is operable to route data from one part of the RAN to another, as well as interfacing with other communication systems. In addition, it performs many of the operation and management functions of a cellular communication system, such as billing. The RAN comprises the base stations, which in UMTS are known as Node Bs, as well as Radio Network Controllers (RNC) which control the Node Bs and the communication over the air interface.
The specifications for UMTS and other 3rd generation cellular communication systems are being standardised in Technical Specifications by the 3rd Generation Partnership Project (3GPP). In order to improve the performance, operation and service provided by UMTS, the Technical Specifications are continuously being updated. For example, in Release 5 of the 3GPP specifications, a new service known as High Speed Downlink Packet Access (HSDPA) has been introduced to offer greater downlink packet throughputs than earlier releases that are reliant upon dedicated or downlink shared channels.
A significantly more efficient and flexible downlink packet data communication is achieved by HSDPA. This is achieved by a combination of features including the introduction of link adaptation in the form of adaptive modulation and coding, the use of incremental redundancy retransmission schemes and a more efficient data scheduling functionality. In particular, the HSDPA architecture splits the Medium Access Control (MAC) layer between the RNC and the base station such that some scheduling of data packets is performed at the base station. The scheduling at the base station allows a much faster scheduling which may take into account the varying propagation conditions for individual mobile stations thereby allowing a more efficient utilisation of the limited air interface resource.
Specifically, the RNC generates data packets known as MAC PDUs (Packet Data Unit) which are aggregated and sent to the base station over the interconnection between these (known as the lub interface) The base stations buffer the PDUs until they are scheduled by the base station scheduler and successfully transmitted over the air interface to a mobile station.
It is clear that in order for such a distributed scheduling operation to function effectively, the control of data transmissions over the Iub must be very efficient. It is thus of the utmost importance that the flow control between the RNC and the base station is efficient and reliable.
In accordance with the Technical Specifications, the base station is the master of the flow control and controls the data exchange. Specifically, the Technical Specifications define a number of data messages that may be used for the flow control. For example, the RNC can request additional buffer space at the base station by transmitting an HS-DSCH CAPACITY REQUEST message. In response, the base station may choose to grant resource using an HS-DSCH CAPACITY ALLOCATION message. This message provides a variety of data that may be used to control the flows.
The grant replaces any existing credits, and may be unsolicited (i.e. it does not have to follow a request from the RNC).
Furthermore, the Node B can make an initial allocation of credits (Initial Window Size) to the RNC when the radio link to the UE is configured.
Furthermore, the HS-DSCH Data Frame used by the RNC to communicate PDUs also comprises a field called User Buffer Size (UBS) which indicates the RNC buffer size for pending PDUs, i.e. the number of PDUs which are pending at the RNC for a given communication.
The Technical Specifications do not define a specific flow control algorithm that must be adopted but rather provides a number of messages that may be used by individual manufacturers to implement their preferred flow control algorithm. It will be appreciated that the performance of such a flow control algorithm is crucial to the performance of the HSDPA service.
Specifically, a flow control algorithm should try to optimise at least the following characteristics:
A minimisation of latency:                Data should not be held at the RNC if there is a possibility of it being scheduled by the base station. In particular, “stalling”should be avoided. Stalling occurs when a communication is scheduled and all its PDUs at the base station are transmitted, but more would have been transmitted if the flow control had delivered additional PDUs from the RNC to the base station.        
A minimisation of the number of PDUs buffered at the base station.                When the mobile station undertakes a handover to a new base station, any PDUs buffered at the current base station are simply discarded. The impact of this on RLC AM (Acknowledged Mode—using retransmissions of lost data packets) radio bearers is one of increased latency—the RLC protocol can recover the loss using retransmissions but this introduces a substantial delay. However, for UM (Unacknowledged Mode—no retransmissions) radio bearers the data is unrecoverably lost.        In certain implementations, base station buffer memory may be a limited resource.        
A minimisation of flow control signalling.                The bandwidth of flow control signalling on the Iub should be acceptable in order to maintain low cost and high throughput.        
However, these requirements tend to be conflicting requirements and a trade off between the requirements is typically required. However, known algorithms tend to be suboptimal and to provide performance and/or trade offs which are undesirable.
Hence, an improved flow control would be advantageous and in particular a system allowing for increased flexibility, low complexity implementation, improved performance, reduced latency, reduced storage of packet data at the base station, reduced signalling overhead and/or improved trade off between conflicting requirements would be advantageous.